Gallium-based superconducting composite

ABSTRACT

The invention provides a superconductor comprising particles made of a superconductive material, and a conductive material. The conductive material is selected to be driven to a superconductive state when in proximity to the superconductive material, and preferably at least includes gallium. An unbroken length of the conductive material is located sufficiently close to a plurality of the particles to be driven to a superconductive state by the superconductive material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application claims priority from U.S. patentapplication Ser. No. 60/424,979, filed on Nov. 8, 2002.

BACKGROUND OF THE INVENTION

1). Field of the Invention

The present invention relates to a superconductor.

2). Discussion of Related Art

The discovery of high critical temperature (Tc) superconducting ceramics(HTS ceramics) has inspired an enormous interest in their application.Conventional niobium alloy superconductors such as NbTi must be cooledto below 10K to achieve useful superconductivity. HTS superconductors,on the other hand, can have Tcs over 100K. Due to the great expense ofcryogenic refrigeration, the HTS ceramics could find much widerapplication in industrial and laboratory devices. Of particular interestare materials which have Tc above 77K, because this is the temperatureof liquid nitrogen, a common and relatively inexpensive refrigerant.

HTS ceramics have not been used in many potential applications becausethey suffer from a number of shortcomings. The most severe problems withthe HTS ceramics are as follows:

1) HTS ceramics are brittle. They are not flexible and thus cannot bemade into wires or other useful shapes. Cracks and boundaries betweenadjacent crystals severely limit supercurrent flow.

2) HTS ceramics are highly anisotropic. Supercurrents preferentiallyflow in certain directions with respect to the crystal lattice, reducingthe maximum current density in randomly oriented multicrystallinepieces.

3) HTS ceramics are strong oxidizing agents. Most metals, such ascopper, lead, tin, aluminum, indium, and niobium, are oxidized bycontact with the ceramic superconductors. Insulating oxide layers impedesupercurrent flow. Only noble metals such as gold, silver palladium andtheir alloys are not oxidized by the HTS ceramics.

A less severe undesirable feature of the HTS ceramics is that they canlose their superconducting properties under certain circumstances. Thesuperconducting structure inside the HTS ceramics has an abundance ofoxygen atoms which are necessary for superconductivity. Heating,grinding, etching, or prolonged exposure to ambient atmosphere or vacuummay liberate the oxygen and destroy superconductivity. Both the oxygencontent and the superconductivity can be restored by annealing the HTSceramic in an atmosphere of oxygen.

It would be an advance in the art of applied superconductivity toprovide a superconducting wire employing HTS ceramics that is ductile,has a high Tc, and has a high critical current density (Jc). Such a wiremust overcome the problems with the HTS ceramics. Prior art HTS ceramicwires made of a combination of HTS ceramic particles in a silver matrixgenerally have poor superconducting properties such as low Jc. Also,bending the prior art wires tends to greatly reduce the Jc. This ishighly undesirable.

There are other ceramic superconductor materials which have some of thesame disadvantages as HTS cuprate materials. For example, the A15 familyof superconductors such as Nb₃Sn are also brittle materials (althoughthey are not anisotropic and relatively nonreactive). Their poormechanical properties have precluded their use in many applicationsrequiring ductility such as wires. This is unfortunate because theygenerally have good superconducting properties such as relatively highTcs, high critical magnetic fields, and high critical current densities.

Other examples of brittle, nonductile superconductors include materialspossessing the NaCl crystal structure (the AB family), Laves phaseceramics, Chevrel phase ceramics, and metallic borides. These materialsmay have superior superconducting properties, but are unusable in manyapplications (e.g., conducting wires) because they are brittle. It wouldbe an advance in the art of applied superconductivity to provideflexible wires made from brittle superconductor materials.

U.S. Pat. No. 5,091,362 to Ferrando discloses a method for forming asilver coating on HTS ceramic particles. U.S. Pat. No. 4,971,944 toCharles et al. teaches a method for electroless deposition of gold ontoHTS ceramic particles.

U.S. Pat. No. 5,041,416 to Wilson describes a superconducting compositematerial. Powders of HTS ceramic and normal metal are mixed and themixture is subjected to heat and high pressure. The composite materialsof Wilson have a relatively low Jc due to reactivity between the HTSceramics and the metal matrix. The wires also have a low Jc if silver isused as the normal metal.

U.S. Pat. No. 5,202,307 to Hayashi describes a superconducting compositematerial having HTS ceramic particles in a metal matrix. The compositematerials of Hayashi have a relatively low Jc due to reactivity betweenthe HTS ceramic particles and the metal matrix and/or due to poorsuperconducting properties of the metal matrix materials.

U.S. Pat. No. 5,194,420 to Akihama describes a composite cupratesuperconductor/metal superconducting material consisting of HTS ceramicparticles dispersed in a matrix of silver. The composite materials ofAkihama will also have a relatively low critical current density due tothe choice of silver as the metal matrix material.

U.S. Pat. No. 5,081,072 to Hosokawa et al. describes a method preparinga HTS superconducting ceramic powder and forming the powder into asuperconducting material. A low Jc is also a problem with the materialsof Hosokawa.

U.S. Pat. No. 5,547,924 to Ito et al. describes a superconductingceramic composite material having HTS ceramic particles in a noble metalmatrix. The composite materials have relatively poor superconductingproperties due to the poor superconducting properties of the metalmatrix materials used.

U.S. Pat. No. 5,132,278 to Stevens et al. describes a cupratesuperconductor wire having continuous filaments of HTS ceramicsurrounded by metal matrix. A noble metal chemically protects the HTSceramic. The wires of Stevens et al. are characterized in that they donot conduct current between wires, and do not rely on thesuperconducting proximity effect.

There exists a need for the HTS ceramic superconducting material that isductile and has a high Jc that is not reduced by bending. Also, thereexists a need for ductile superconducting materials made from brittlesuperconductors that have a high Jc high Tc, and high ductility.

SUMMARY OF THE INVENTION

The invention provides a superconductor comprising particles made of asuperconductive material, and a conductive material. The conductivematerial is selected to be driven to a superconductive state when inproximity to the superconductive material, and preferably at leastincludes gallium. An unbroken length of the conductive material islocated sufficiently close to a plurality of the particles to be drivento a superconductive state by the superconductive material.

The superconductive material may be a planar diboride.

The planar diboride is preferably magnesium diboride.

The invention also provides a method of making such a superconductor.The invention also provides a method of making a wire of such asuperconductor. An elongate member may be assembled from the particlesand the conductive material, and be drawn into a wire.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example with reference tothe accompanying drawings wherein:

FIG. 1 is a cross-sectional view of an embodiment of the presentinvention having ceramic superconducting particles disposed in a metalmatrix material;

FIG. 2 is a graph illustrating the variation of the superconducting gapmagnitude near a boundary between a superconductor and a metal;

FIG. 3 is a graph illustrating the variation of the superconducting gapmagnitude near a boundary between a superconductor and a metal having alow λ value;

FIG. 4 is a graph illustrating the variation of the superconducting gapmagnitude near a boundary between a superconductor and a metal having ahigh λ value;

FIG. 5 illustrates the extent of the superconducting proximity effectaround a particle of superconducting material disposed within a metalmatrix;

FIG. 6 shows how a number of superconducting particles disposed in ametal matrix provide a continuous supercurrent path through the matrixmaterial;

FIG. 7 is a graph of critical current density (Jc) versus volume %matrix material for four different matrix materials (silver, aluminum,indium, and lead), the plot illustrates that high metals are betterproximity superconductors than low metals. The ceramic superconductormaterial used was Nb₃Sn;

FIG. 8 is a graph of critical temperature (Tc) versus volume % matrixmaterial for four different matrix materials (silver, aluminum, indium,and lead);

FIG. 9 is a graph of E-field vs. current density, illustrating themeaning of an n-value for a superconducting material, as is known in theart;

FIG. 10 is a graph of n-value versus volume % matrix material for fourdifferent matrix materials (silver, aluminum, indium, and lead);

FIG. 11 is a graph of Jc versus normalized electron-phonon couplingconstant for different matrix materials;

FIG. 12 is a cross-sectional view of an embodiment of the presentinvention where the superconductor particles are coated with achemically compatible metal coating to chemically protect thesuperconductor particles from the metal matrix materials;

FIG. 13 is a graph of critical density versus normalized electron-phononcoupling constant for the embodiment of FIG. 12 having a silver coatingsurrounding each superconductor particle;

FIG. 14 is a graph illustrating the prevailing, incorrect model of thesuperconducting proximity effect in a three-layer system;

FIG. 15 is a graph illustrating the current model of the superconductingproximity effect in a 3-layer system, as developed by the presentinventor;

FIG. 16 is a cross-sectional view of an embodiment of the presentinvention using HTS ceramics as the superconducting particles. Theparticles are coated with a thin coating of noble metal that is notoxidized by the HTS ceramics particles;

FIG. 17 illustrates how the embodiment of FIG. 16 provides a continuoussupercurrent path through the matrix material.

FIG. 18 is a side view of a sheet of protective material withsuperconductor particles in granular form deposited thereon;

FIG. 19 is a view similar to FIG. 18 wherein the superconductorparticles are spread over the sheet;

FIG. 20 is a view similar to FIG. 19 after another sheet of protectivematerial is located on the superconductor particles;

FIG. 21 is a perspective view of a composite sheet which is formed aftera sheet of conductive material is located on the top protective sheet ofFIG. 20;

FIG. 22 is a perspective view of an elongate member that is formed byfolding, or rolling the composite sheet of FIG. 21;

FIG. 23 is a side view illustrating how the elongate member of FIG. 22is rolled into a wire;

FIG. 24 is a cross-sectional view of the elongate member before beingrolled;

FIG. 25 is a cross-sectional view of the wire after being rolled;

FIG. 26 is a cross-sectional view on 26-26 of the wire in FIG. 25illustrating how the conductive material is induced to a superconductivestate by the superconductor particles;

FIG. 27 is a cross-sectional view illustrating a two component wire thatis made according to a method similar to that of FIG. 25;

FIG. 28 is a cross-sectional view on 28-28 of the wire of FIG. 27;

FIG. 29 is a cross-sectional view of a three-component granule used forpowder-in-tube manufacture of wire;

FIG. 30 is a cross-sectional view of an elongate member which issymbolizing a plurality of the granules of the FIG. 29;

FIG. 31 is a cross-sectional view of wire being rolled from the elongatemember of FIG. 30;

FIG. 32 is a cross-sectional view of a two component granule used formaking three-component wire in a powder-in-tube manufacturing process;

FIG. 33 is a cross-sectional view of an elongate member which isassembled utilizing a plurality of the granules of FIG. 32;

FIG. 34 is a cross-sectional view of wire that is rolled out of theelongate member of FIG. 33;

FIG. 35 is a cross-sectional view of a superconductor particle havingexposed outer surfaces;

FIG. 36 is a cross-sectional view of an elongate member that isassembled from a plurality of the superconductor particles of FIG. 35and other components for purposes of making wire according to athree-component powder-in-tube method;

FIG. 37 is a cross-sectional view of a wire that is drawn from theelongate member of FIG. 36;

FIG. 38 is a cross-sectional view of a two component granule used formaking a two component wire in a powder-in-tube process;

FIG. 39 is a cross-sectional view of an elongate member which isassembled from a plurality of the granules of FIG. 38;

FIG. 40 is a cross-sectional view of a wire that is drawn from theelongate member of FIG. 39;

FIG. 41 is a cross-sectional view of a superconductor particle havingbare outer surfaces that is used for making a two component wire in apowder-in-tube process;

FIG. 42 is a cross-sectional view of an elongate member which isassembled from a plurality of the granules of FIG. 41 and othercomponents for making a two component wire;

FIG. 43 is a cross-sectional view of a wire that is drawn from theelongate member of FIG. 42; and

FIG. 44 is a graph illustrating the increased performance ofgallium-based superconductor/metal matrix composite (SMMC) tape.

DETAILED DESCRIPTION OF THE INVENTION

Materials and Compositions

FIG. 1 shows a magnified view of a composite material 20 according tothe present invention. The composite material 20 has superconductorparticles 22 made of brittle superconducting ceramic disposed in a metalmatrix material 24. The superconductor particles 22 can have a widerange of sizes, and the sizes of the particles 22 do not need to beuniform. The particles preferably have sizes generally within the rangeof 1.5 nanometers to 10 microns in diameter, for example. The particles22 are made of a brittle superconducting ceramic, such as an A15compound, AB family superconductors, Laves phase superconductors,Chevrel phase superconductors, metallic borides or the like. Preferably,the particles are made of a material having a relatively high criticaltemperature (e.g., greater than 100 kA/cm²). Also preferably, theparticles have high critical magnetic fields (e.g., greater than 5T). Ingeneral, the superconductor particles preferably have robustsuperconducting properties.

There are preferably no insulating contaminants between thesuperconductor particles 22 and the metal matrix material 24. A thinlayer of grease, oxide or any other insulating material between thesuperconductor particles and metal matrix material can seriously degradethe superconducting properties of the composite material 20 by impedingsupercurrent flow through the superconductor particle/metal matrixinterface.

The particles 22 preferably have dimensions larger than thesuperconducting coherence length of the superconducting ceramic.Preferably, the superconductor particles have dimensions of about 3-500times the superconducting coherence length of the superconductorparticles, more preferably, the superconductor particles are about 5 to10 times larger than the superconducting coherence length of the ceramicmaterial. For most superconductor particle materials, it is preferablefor the intrinsic superconductor particles to have dimensions less than10 microns. The best size range depends upon the temperature at whichthe composite material is used, the λ of the metal matrix material, thesuperconductor coherence length of the superconductor particles, theproximity effect decay length of the metal matrix material, and theinelastic mean free path of the metal matrix material (explained below),and possibly other factors.

For example, the A15 superconducting compounds (e.g., Nb₃Sn, Nb₃Ge,Nb₃Al, V₃Ga, V₃Si, V₃Al, V₃In, Nb₃Ga, V₃Ge, Nb₃Ge, Nb₃Si, Ta₃Pb, Ta₃Auand Mo₃R) generally have superconducting coherence lengths ofapproximately 2-3 nanometers. Therefore, for A15 compounds, thesuperconductor particles preferably have dimensions greater than 2-3nanometers. For A15 compounds, particle sizes are preferably in therange of 100-5000 nm, or more preferably 10-500 nanometers. Larger sizesare also possible, but are typically less preferred because they mayproduce composites having less than optimal superconducting andmechanical properties.

Table 1 below lists several candidate materials useful for the intrinsicsuperconductor particles, their coherence lengths and their criticaltemperatures.

TABLE 1 Candidate Superconductor Materials Family/ Maximum CoherenceSuperconductor Phase Critical Temperature Length A15 A15 11-25 K 2-3 nmCompounds MoC AB 14.3 K NbN AB 17.3 K 4.0 nm ZrN AB 10.7 K CaRh₂ Laves6.4 K CaIr₂ Laves 6.2 K ZrV₃ Laves 9.6 K 4.5 nm HfV₂ Laves 9.4 K 3.9 nmMo₆S₈ Chevrel 1.6 K Cu₂Mo₆S₈ Chevrel 10.7 K Yb_(1.2)Mo₆S₈ Chevrel 8.7 KLaMo₆S₈ Chevrel 6.6 K 7.8 nm LaMo₆S Chevrel 11.7 K Pb_(0.9)Mo₆S_(7.5)Chevrel 15.2 K 2.0 nm PbMo₆S₈ Chevrel 12.6 K 2.3 nm BrMo₆S₈ Chevrel 13.8K MgB₂ Metallic 40.2 K 5 nm Borides

Ceramics useful for the intrinsic superconductor particles are notnecessarily limited to those listed in Table 1.

The metal matrix material 24 is preferably made of a ductile metal(elemental metal, metal alloy, or metal mixture) that is susceptible tothe superconducting proximity effect. In order to be susceptible to theproximity effect, the metal matrix material preferably has a highelectron-boson coupling coefficient, typically an electron-phononcoupling coefficient, λ (a unitless number). The metal matrix materialmust have a λ greater than 0.2. More preferably the metal matrixmaterial has a λ greater than 0.5, and most preferably the metal matrixmaterial has a λ greater than 1.0. All else being equal, the higher theλ, the better. This is because the susceptibility to the proximityeffect increases with λ. Table 2 below shows candidate matrix materialsand their electron-phonon coupling coefficients.

TABLE 2 Candidate Metal Matrix Materials Metal Electron-Phonon Coupling,λ Lead 1.55 Tin 0.72 NbTi 0.92 Niobium (Nb) 1.22 Mercury 1.62 Tantalum0.69 Titanium 0.8 Vanadium 0.8 Indium 0.81 Ti_(0.9)Bi_(0.1) 0.78Pb_(0.4)Ti_(0.6) 1.38 Pb_(0.9)Bi_(0.1) 1.66 Pb_(0.75)Bi_(0.25) 2.76Pb_(0.7)Bi_(0.3) 2.01 Lead/Indium Alloys >1.5 Lead/Bismuth Alloysgenerally 2.3

Metals useful for the metal matrix material are not necessarily limitedto those listed in Table 2. Many of the metal matrix materials in Table2 are elemental materials (e.g., Lead, Tin, Niobium). These elementalmaterials are typically Type I superconductors and generally are notsuitable for use in high magnetic field applications. For high magneticfield applications, Type II superconductors (e.g., metal alloys) shouldbe used for the metal matrix materials. Examples of Type II alloys areNbTi and Lead-Bismuth alloys. Also, it is known that some elementalmetals act as Type II superconductors if under sufficient mechanicalstress. Stressed Niobium, for example, acts as a Type II superconductorand can be used as a metal matrix material in high magnetic fieldapplications, though well annealed, unstressed niobium is a classic TypeI superconductor.

Also, the metal matrix material preferably has a long proximity effectdecay length. The proximity effect decay length is a fundamentalmaterial property. The decay length is a result of dephasing of electronand hole wave functions as they travel through the matrix material (dueto different electron and hole velocities). The decay length istypically not as important as λ because most candidate materials havesimilar decay lengths. Table 3 shows decay lengths for selectedcandidate materials. It is noted that the decay length is inverselyproportional to absolute temperature.

TABLE 3 Decay Lengths for Selected Matrix Materials Proximity EffectDecay Length at 77 K in Metal nm Lead 29 Tin 30 NbTi 22 NbTi, (Nb) 22Mercury 25 Indium 28 Lead/Bismuth Alloys 25-28

The superconductor particles 22 preferably have a critical temperaturehigher than the critical temperature of the metal matrix material 24.

FIG. 2 shows a diagram illustrating the superconducting proximity effectas it occurs at a boundary 27 between a superconductor 26 and a metal 28susceptible to the proximity effect. A superconducting gap magnitudeprovides a measure of superconductivity. The superconducting gapmagnitude is proportional to the critical temperature and the criticalcurrent density in a superconducting material. In the metal the gapmagnitude 29 is relatively high close to the boundary 27, and decreaseswith distance from the boundary. The metal 28 is superconducting closeto the boundary 27 due to contact with the superconductor 26. Thesuperconducting gap magnitude in the metal decreases with increasingdistance from the boundary 27. The characteristic length of the decay ofthe superconducting gap magnitude in the metal is the proximity effectdecay length.

FIG. 3 shows a diagram illustrating the superconducting gap magnitude inthe case where the metal has a low electron-phonon coupling coefficient(λ less than 0.2). The gap magnitude 29 in the metal is relativelysmall, and therefore the metal has relatively small critical currentdensity. The metal has poor superconducting properties due to the low λvalue. Examples of materials with low λ include silver with λ=0.14, goldwith λ=0.14, and copper with λ=0.08. These materials are very goodconductors, but have poor superconducting properties. Therefore, theyare preferably not used as the metal matrix material.

FIG. 4 shows a diagram illustrating the superconducting gap magnitude inthe case where the metal has a high electron-phonon coupling coefficient(λ greater than 1.0). The gap magnitude 29 in the metal is relativelylarge compared to the gap magnitude in FIG. 3. The high λ results in themetal having a larger critical current density extending deeper into themetal layer. The proximity decay length of the metal also plays a role.

In the present invention (e.g., the embodiment shown in FIG. 1), thesuperconductor particles 22 are superconducting at the operatingtemperature, and the metal matrix material is induced superconductingthrough the superconducting proximity effect, even though itssuperconducting transition temperature may be below the operatingtemperature. FIG. 5 shows a single isolated superconductor particle 32disposed in a block 34 of metal matrix material. When cooled below thecritical temperature of the particle 32 (but not necessarily below thecritical temperature of the metal matrix material block 34), thesuperconductor particle 32 causes portions of the block close to thesuperconductor particle to become superconducting due to the proximityeffect. Circle 36 illustrates the approximate range of the proximityeffect. The circle 36 is relatively large if the metal matrix materialhas a high λ value and long proximity decay length; the circle isrelatively small if the metal matrix material has a small λ value and/orshort proximity decay length. If the metal matrix material block is madeof silver, gold, or copper, for example, the circle 36 will extend onlya very small distance beyond the surface of the superconductor particleand the block will have essentially no useful superconductivity.

FIG. 6 shows an idealized embodiment of the present invention in whichall the superconductor particles 22 are arranged linearly in the metalmatrix material 34. Circles 36 illustrate the approximate range of thesuperconducting proximity effect within which superconducting gap has amagnitude great enough to provide a useful critical current density. Theparticles 22 are close enough so that the circles imply that theparticles 50 are coupled by a continuous superconducting path due to theproximity effect. The path 38 can pass through the superconductorparticles 22 and the superconducting regions of the metal matrix, or canpass only through the superconducting regions of the metal matrixmaterial 34. Very large superconductor particles (with dimensions muchgreater than the range of the proximity effect) are usually notpreferred due to ‘voids’ in the metal matrix material that were largerthan the range of the proximity effect. Generally, the superconductorparticles are preferably small enough and numerous enough so thatsubstantially all the metal matrix material is affected by the proximityeffect. Of course, the optimum size of the superconductor particlesdepends somewhat on the volume percent of superconductor particlematerial versus matrix material, and the temperature at which thecomposite material is used.

In FIG. 1, the superconductor particles 22 are close enough and numerousenough so that essentially the entire volume of the metal matrixmaterial is induced into a superconducting state by the proximity effectwhen cooled below the critical temperature of the superconductorparticles 22. Therefore, the entire volume of the composite material issuperconducting when cooled below the critical temperature of thesuperconductor particles 22.

An important consideration in the present invention is the relativevolume of the superconductor particles and the metal matrix material.The optimal percentages (measured by volume) of superconductor particlesand metal matrix material depends greatly on the mechanical andsuperconducting properties of the materials used, and the desiredmechanical and superconducting properties of the composite material. Forexample, if very high ductility is desired of the composite material,then a high percentage of the very ductile metal matrix material shouldbe used. Generally, the optimal volume percent of metal matrix materialincreases with increasing λ.

Also, the temperature at which the composite is to be used has bearingon the composite design. The proximity effect decay length increaseswith decreasing temperature. Therefore, for a composite used at very lowtemperatures, the superconductor particles 22 can be located relativelyfar apart. If the composite is to be used at relatively hightemperatures, then the superconductor particles 22 are preferablylocated relatively close together.

There are many possible combinations (e.g., thousands) of superconductorparticle materials and metal matrix materials within the scope of thepresent invention. Each possible combination may also be improved oroptimized by selecting the best superconductor particle volume/matrixmaterial volume ratio. Also, each material combination may be improvedor optimized by selecting the best size range for the superconductorparticles. Further, there may be special chemical compatibility issuesfor certain combinations for superconductor particle materials and metalmatrix materials. In general, chemical reactivity between the particlesand metal matrix material should be avoided, particularly if thereactivity is destructive to the superconducting properties of thesuperconductor particles, or if the reactivity is destructive to thesuperconducting properties of the metal matrix material, or if thereactivity degrades the electrical contact between the superconductorparticles and the metal matrix (e.g., by producing an insulating layerat the particle/matrix interface).

FIG. 7 shows critical current densities for several different materialcombinations at 4.2K. The different material combinations are Nb₃Snsuperconductor particles dispersed within silver, aluminum, indium andlead. A cross 40 represents the critical current density for purecompacted Nb₃Sn superconductor particles (e.g., a powder-in-tube Nb₃Snwire). In all the composites represented in FIG. 7, the Nb₃Snsuperconducting particles were −325 mesh, which corresponds to particlesizes in the range of about 1-40 microns. It is possible the criticalcurrent densities of the composites could be improved by usingsuperconductor particles with more uniform sizes. The critical currentdensity was determined by the standard 0.1 μV/cm electric fieldcriterion.

FIG. 7 demonstrates that a composite of Nb₃Sn particles in a silvermatrix has a dramatically reduced supercurrent carrying capacitycompared to pure compressed Nb₃Sn particles. This is because silver hasa very low λ and is therefore only very weakly susceptible to thesuperconducting proximity effect.

Similarly, the composite material which consists of Nb₃Sn superconductorparticles embedded in an aluminum matrix has a reduced Jc relative tothe pure Nb₃Sn particles because aluminum has a relatively low λ of0.43. However, the composite which consists of Nb₃Sn superconductorparticles embedded in an indium matrix has an improved Jc relative topure Nb₃Sn, Nb₃Sn/silver, or Nb₃Sn/aluminum materials. FIG. 7 indicatedthat the optimum amount of indium is about 10-35% by volume. Indiumimproves the Jc of the composite because indium, with a λ of 0.81, hasrelatively high susceptibility to the superconducting proximity effect.Similarly, lead also improves the Jc of the composite because lead, witha λ of 1.55, also has a high susceptibility to the superconductingproximity effect. However, embedding the Nb₃Sn superconductor particlesin the lead matrix does not improve the Jc as much as would be expectedfrom its λ value. This is because the tin (Sn) within the Nb₃Sn tends tooffset the Jc improvement provided by the high λ of lead. This effect ofchemical reactivity is worsened by heating the composite material.Several possibilities for avoiding this effect are discussed below.

FIG. 8 shows critical temperatures (Tcs) for some of the same compositematerials plotted in FIG. 7. Bulk Nb₃Sn has a Tc of 18K. However, the Tcof pure compacted Nb₃Sn powder is about 10K, which is indicated in thegraph at 42. This low value for Tc reflects the poor inter-particlecontact between the Nb₃Sn superconductor particles in a compactedmaterial. FIG. 8 shows that the addition of silver or aluminum to Nb₃Snparticles dramatically decreases the Tc compared to pure compacted Nb₃Snparticles. Aluminum has a Tc of 1.4K and silver is not superconductingat any temperature. By comparison, indium, which has Tc of 3.2Kdramatically increases the Tc of the Nb₃Sn/indium composite to over 10K.This is a striking result considering that Indium has a Tc less than10K. Indium increases the Tc of the composite because indium has arelatively high λ of 0.81 and is thus quite susceptible to thesuperconducting proximity effect. Similarly, lead increases the Tc ofthe Nb₃Sn/lead composite because lead has a λ=1.55. The effect of thehigh λ of lead is offset due to the chemical reactivity between lead andNb₃Sn. It is expected that further increases in the critical temperatureof Nb₃Sn/lead composites would

be realized if not for the dissociation of tin from Nb₃Sn when in thepresence of lead.

The composites of the present invention can be adjusted to have desired‘n-values’. FIG. 9 illustrates the concept of an n-value as is known inthe art. The n-value of superconducting material describes the sharpnessof the conversion to the normal state when the critical current densityis exceeded. Generally, the n-value is defined by the relation:V/Vc=(I/Ic)^(n),

where I and V are the current and voltage in the wire. Vc is theelectric field criterion standard (typically 0.1 μV/cm), and Ic is thecritical current of the composite which is defined as the magnitude ofcurrent passing through the composite which results in an electric fieldof at least Vc existing in the composite. An n-value of 1 corresponds toOhms Law for a normal resistive metal. The n-value can be an importantcharacteristic for certain superconductor applications. For example, infault-current limiters for electrical power distribution application, itis highly desirable to have a very high n-value so that power surges areefficiently attenuated. Such devices are operated very close to Ic. Acurrent surge exceeds Ic and pushes the device into normal conduction,thereby absorbing the power surge. The present invention can provide acomposite for fault current limiters that have a very high n-value(generally at the expense of a lower Jc or Tc). Conversely,superconducting wire for electric motors should have a relatively lown-value so that large, sudden changes in motor impedance are notproduced by current surges and so that changes in motor load do notresult in current surges. The present invention provides flexibility sothat specific n-values can be provided.

FIG. 10 plots n-values for several Nb₃Sn/metal matrix composites madewith silver, aluminum, indium and lead matrix materials. The n-value fora pure compressed Nb₃Sn superconductor particles is about 10. EmbeddingNb₃Sn superconductor particles in a matrix of silver or aluminum reducesthe n-value compared to pure compressed Nb₃Sn particles. Embedding Nb₃Snsuperconductor particles in a matrix of 10-35% indium by volumeincreases the n-value. Embedding Nb₃Sn superconductor particles in amatrix of lead increases the n-value. The n-value for lead is measuredto be about 70. The measured n-values for Nb₃Sn/lead composites are notconsistent because of variations caused by the reactivity between theNb₃Sn and lead. Since silver, aluminum, indium and lead have differenteffect on the n-value, a desired n-value can be produced by using acombination (mixture) of these metals for the matrix material. Forexample, a combination of lead and silver will produce a wire having areduced n-value compared to a wire with a matrix of pure lead. Byselecting appropriate ratios of different metal matrix materials, adesired n-value is provided.

FIG. 11 shows a graph of the normalized electron-boson coupling constantversus critical current density. The normalized electron-boson couplingconstant includes the effect of the screened electron-electron Coulombrepulsion μ*. For most materials, the screened electron-electron Coulombrepulsion is about 0.1.

The normalized electron-boson coupling constant is given by(λ−μ*)/(1+λ).

The normalized electron-boson coupling constant is proportional to thesuperconducting gap magnitude in a metal adjacent to a superconductorand therefore is a more accurate measure of proximity effectsusceptibility than the electron-boson coupling coefficient λ. Thenormalized electron-boson coupling constant values for silver, aluminum,indium, and lead are about 0.03, 0.19, 0.38 and 0.55, respectively. FIG.11 illustrates that Jc increases strongly with increasing λ andnormalized electron-boson coupling constant. These results suggest thatvery high λ materials such as lead-bismuth alloys will provide very highJc's significantly exceeding 100 kA/cm² if added to the composite insufficient quantity. Also, these results suggest that, if all otherfactors are constant, higher normalized electron-boson couplingconstants are always better.

FIG. 11 also demonstrates that too much matrix material strongly reducesthe Jc of the composite material unless the normalized electron-bosoncoupling is very high. The higher the normalized electron-boson couplingconstant of the matrix material, the more matrix coupling constant ofthe matrix material, the higher the optimal volume % of matrix materialfor maximum Jc. In other words, for maximum Jc, matrix materials withhigher reduced phonon coupling should comprise higher volume % of thecomposite material. For matrix materials with λ in the range of about1.5-2, it is estimated that the matrix material should comprise about25-50% of the composite by volume.

The reaction between lead and Nb₃Sn causes degradation of thesuperconducting properties of Nb₃Sn/lead composites. Also, thesuperconducting properties are unstable and may deteriorate when thecomposite is exposed to heat which promotes the reaction. Therefore,Nb₃Sn/lead does not provide a good superconducting composite. As notedabove, the tin dissociates from the Nb₃Sn and dissolves in the lead.

Generally the A15 compounds tend to be somewhat unstable. If a componentof the A15 compound is soluble in the matrix material or binds with thematrix material, then this can promote the dissociation of the A15compound. For this reason it is best to use A15 compounds havingconstituents which are insoluble in the metal matrix material or have alow affinity for the metal matrix material. This helps prevent the A15compound from dissociating and degrading the properties of thecomposite. For example, if lead is used as a matrix material, other A15compounds not containing tin can be used.

Most generally, any chemical reactivity between the superconductorparticles and metal matrix material should be avoided if it adverselyeffects the superconducting properties of the superconductor particles,the metal matrix materials, or the superconductor particle/metal matrixinterface. In all the embodiments of the present invention, theparticles are made of superconducting ceramics. Any metal matrixmaterial that promotes the chemical breakdown of the superconductorparticles should be avoided. Lead is an example of the metal matrixmaterial that promotes the chemical breakdown of Nb₃Sn.

FIG. 12 shows an alternative embodiment of the present invention whichuses Nb₃Sn superconductor particles 46 and lead as the matrix material48. Each Nb₃Sn superconductor particle 46 has a thin metal film coating44 which prevents chemical reactions from occurring between the Nb₃Snsuperconductor particles 46 and lead matrix material 48. The coating ischemically compatible with the particles 46 and is chemically compatiblewith the metal matrix material 48. The coating 44 can be made of manydifferent materials but is preferably metallic and electricallyconductive. The coating can be silver, for example. The coating 44should be thin enough so that the proximity effect from the Nb₃Snsuperconductor particles 46 can reach the lead matrix material. This ispreferably accomplished by making the coating 44 as thin as possiblewhile still providing chemical isolation between the Nb₃Snsuperconductor particles and lead. More specifically, the coating shouldbe substantially thinner than the inelastic electron mean free path(MFP) of the coating material at the temperature that composite materialis used (e.g., the critical temperature of the particles). For silver,which does not react with Nb₃Sn the MFP is about 600 nanometers at 18Kelvin, the Tc for Nb₃Sn. More preferably, the coating thickness is lessthan ½ or ¼ of the electron MFP at the Tc of the superconductingparticles.

Preferably, the metal coating is thinner than an electron mean free pathin the metal coating material at 4.2 Kelvin. Also, the metal coating ispreferably thinner than a proximity effect decay length of the metalcoating material at 4.2 Kelvin.

It is noted that metals with a long election MFP tend to have low λvalues and are therefore are not susceptible to the proximity effect.However, a long mean free path in the coating increases the‘penetration’ of the proximity effect through the coating. A coatingmaterial with a high λ and short MFP will tend to reduce the proximityeffect in the matrix, but in this case the coating itself will be moresusceptible to the proximity effect. Generally, if a coating is appliedto the superconductor particles, a balance is preferably providedbetween the coating thickness, electron mean free path, and λ value. Forexample, a very thick silver coating (much thicker than the silver MFP)would be undesirable because the silver is not susceptible to theproximity effect, and the silver coating would reduce the proximityeffect in the matrix. In general, if low λ/high MFP coating is used, itshould be as thin as possible while still preventing chemical reactionsbetween the particles and matrix materials. For example, 5-10 nanometersof silver can be sufficient for preventing certain chemical reactions,and the MFP for silver is much greater than 10 nm at temperaturesnecessary for superconductivity. If a high/low MFP coating is used, theoptimal thickness depends on the coating λ and the MFP, as well as themetal matrix material λ.

Table 4 shows several metals which can be used for a metal coating incase where chemical incompatibility exists between the superconductorparticles and the metal matrix material. The MFP increases withdecreasing temperature. The MFP depends upon the microstructure of thematerial and so may vary from the values shown (e.g., depending on theprocess used to make the coating).

TABLE 4 Candidate Metal Coating Metals Metal Inelastic mean free path at77 Kelvin Silver 285 nm Gold 205 nm Aluminum 131 nm Copper 328 nm Tin100 nm Lead 141 nm

FIG. 13 shows experimental results for four composites having Nb₃Snsuperconductor particles coated with about 100 nanometers of silver. Thecoated superconductor particles are disposed in silver, aluminum, indiumor lead matrix materials (the silver coating is not considered part ofthe matrix material). The Jc's for the four composites are indicated byX's. In each of the four composites, the silver coating comprises about5% of the composite volume. The matrix material (silver, aluminum,indium, or lead) comprises about 15% of the composite volume. The silvercoating prevents chemical reactions between the Nb₃Sn superconductorparticles. The important point here is the high-λ metals (e.g., indium,lead) increase the Jc of the composite compared to low-λ matrixmaterials. This is remarkable considering that the matrix is separatedfrom the Nb₃Sn particles by 100 nanometers of silver. The proximityeffect extends through the silver coating. If the proximity effect didnot extend through the 100 nm silver coating, the Jc for the indium andlead matrix composites would be the same as for the Nb₃Sn/silver matrixcomposite or essentially zero (i.e., less than 10 A/cm²). The fact thatindium and lead dramatically increase the Jc while not in directphysical contact with the superconductor particles is proof that theproximity effect extends through the silver coating. It is noted thatthe silver coating does negatively impact the wire performance somewhat.This negative impact is preferably minimized by reducing the thicknessof the silver coating (e.g., to less than 120 nanometers). For example,a silver coating of 100 nanometers is unnecessarily thick, but can beused.

Prior to the development of the present invention, it was mistakenlyassumed by many in the field of superconductivity that the proximityeffect could not extend through a thin layer of low-λ metal (such as thenoble metals in Table 4). FIG. 14 illustrates the prevailing butincorrect understanding of what was expected in such a ‘three-layer’system. The superconductor 26 is separated from a high-λ metal 62 by athin layer of low-λ metal 60. The low-λ metal has a very smallsuperconducting gap magnitude in the high-λ metal. This model of theproximity effect in the three-layer system is absolutely incorrect.

FIG. 15 illustrates the correct model of the superconducting proximityeffect in a three-layer system. Remarkably and surprisingly, thesuperconducting gap magnitude 66 rebounds in the high-λ metal to a valuepossibly much higher than the gap magnitude in the low-λ metal 60. Theability of the gap magnitude to rebound in a three-layer system has beenconfirmed in experiments performed by the present inventor. Of course,in order for the superconducting gap to rebound, the high-λ metal musthave a λ substantially higher than the λ of the low-λ metal. Therebounding effect provides the physical basis of operation of thecomposite materials of the present invention which employ a low-λ metalcoating surrounding of the superconductor particles. For example, it isthe rebounding effect which provides the improved Jc in the compositematerial having silver-coated Nb₃Sn superconductor particles in a leadmatrix (explained with reference to FIGS. 12 and 13). It is important tonote that the low-λ metal is minimized by making the low-λ coating asthin as possible, and by using a low-λ coating with a long MFP.

It is noted that, although the foregoing embodiments of the presentinvention have been primarily explained with reference to Nb₃Sn as thesuperconductor particle material, the superconducting particles can bemade of many other superconducting compounds and ceramics. Also, thesuperconducting particles can be made of a mixture of materials (i.e.,each superconductor particles within the same composite can be made ofdifferent superconducting compounds or ceramics). The choice ofsuperconductor particle material and matrix material is an importantone, and depends upon the mechanical, electrical and chemicalcharacteristics of the superconductor particles and matrix materials aswell as the desired properties of the composite material (e.g.,ductility, critical current density, n-value, critical temperature,chemical reactivity). For many material combination, optimal volumeratios and superconductor particle sizes (e.g., producing the highestJc, the highest Tc, the highest/lowest n-value, highest ductility) canbe found by routine experimentation using the guidance and teaching ofthe present description.

The composites of the foregoing embodiments are in the form of singlefilament wire and are simple to manufacture using powder-in-tube methodsknown in the art. First, a powder of superconducting particles isthoroughly mixed with a metal matrix material. Each constituent materialin the composition is preferably clean and free of contaminants.Preferably, the superconductor particles and matrix material are handledin an inert atmosphere. The mixture is placed within a billet such as acopper, silver, aluminum, or rubber tube, though any non-reactive tubewill work also. Next, the powder mixture is compressed in the billet,for example using a cold isostatic press. The compression step removesthe majority of the voids from the composite material. Optionally, thecompression step is performed in vacuum so that the void volume isminimized. Next, the billet containing the fused material is drawn in toa wire using known wire drawing techniques. Annealing steps may berequired between the drawing steps. It is noted that the typical powdermetallurgical techniques may leave voids comprising about 10-20% of thecomposite volume.

The relative volumes of the superconductor particles and metal matrixmaterial is determined by the amount of the material originally mixedtogether.

If it is desired to provide the superconductor particles with a noblemetal coating (e.g., a silver coating), then this coating is preferablyapplied before mixing the superconductor particles with the metal matrixmaterial. The coating can be applied using known chemical or physicaldeposition techniques.

An alternative method for making composite material begins by coatingthe superconductor particles with a coating of metal matrix material.The thickness of the coating is preferably controlled accurately. Theinterface between the superconductor particles and the metal matrixmaterial coating is preferably clean and free of grease, oxides and anyother insulating contaminants. Next to form a wire from the compositematerial, the metal matrix coated superconductor particles are placed inthe billet, compressed, and drawn into a wire. The relative volumes ofthe intrinsic superconductor particles and metal matrix material isdetermined by the size of the superconductor particles and the thicknessof the metal matrix material coating. Of course, the composite materialof the present invention can be made into any other shape such as bars,rods, sheets, or plates.

A very special set of embodiments of the present invention employ thehigh temperature superconductor (HTS) ceramic materials (HTS ceramics).In the present specification, HTS ceramics are defined as having acritical temperature greater than 30 Kelvin. Several examples ofsuitable HTS ceramics are given in Table 5. The HTS ceramic YBa₂Cu₃O₇ ispreferred for many applications (e.g., wire in high magnetic fields)because of its high flux pinning strength and high Tc.

TABLE 5 Ceramic Superconductors HTS Ceramic Critical Temperature, T_(c)Bi₂Sr₂Ca₂Cu₃O₁₀ 105 K Bi₂Sr₂CaCu₂O₈  85 K (BiPb)₂Sr₂Ca₂Cu₃O₁₀ 110 KTl₂Ba₂Ca₂Cu₃O₁₀ 125 K HgBa₂CaCu₂O₆ 135 K HgBa₂Ca₂Cu₂O₆ 135 K Tl₂Ba₂CuO₆ 80 K La_(1.8)Sr_(0.15)CuO₄  40 K Tl₂Ba₂CaCu₂O₈ 105 K

The HTS ceramics listed above are strong oxidizing agents and willoxidize on contact all but the most noble (nonreactive) metals. Silver,gold, and palladium, for example, are not oxidized by contact with theHTS ceramics. However, these metals have very low electron-phononcoupling coefficients and are not very very poor proximitysuperconductors. Table 6 lists the electron-phonon coefficients for somenoble metals not oxidized by the HTS ceramics.

TABLE 6 Electron-phonon Coefficients for Some Noble Metals MetalElectron-phonon coupling, λ Silver   0.14 Gold   0.14 Palladium <0.10(approximate)

Generally, the noble metals have very low electron-phonon couplingcoefficients. This explains why prior art composite superconductor wiresmade with HTS ceramic particles in a silver matrix have relatively lowJc values and are sensitive to mechanical stress. In these wires, thesupercurrent tends to flow directly between HTS ceramic particles wherethey are superconducting). Consequently, supercurrent paths aredisturbed when the material is flexed. Silver, gold, and palladium arevery weakly susceptible to the proximity effect due to their very low λvalues.

If HTS ceramic particles are disposed in a matrix of a non-noble metal(e.g., lead, indium, tin, NbTi or any other metals known to react withthe HTS ceramics), an insulating metal oxide coating forms at theinterface between the HTS ceramic particles and the non-noble metal. Themetal oxide coating almost completely blocks the proximity effects andblocks supercurrent from flowing between adjacent HTS ceramic particles.A composite material having HTS ceramic particles in a matrix ofoxidizable metal is almost completely useless as a superconductor due tothe insulating metal oxide coating. The oxidizable non-noble metalscannot be combined with HTS ceramic particles in a composite material tomake a useful superconducting composite employing the proximity effect.

There is presently no known material that resists oxidation by the HTSceramics and has a high λ (e.g., λ greater than 1.0). If such a metal isfound or created, it could be combined with HTS ceramic particles in acomposite to make a very high quality superconducting composite materialwith Tc and high Jc.

FIG. 16 shows a view of a superconducting composite material accordingto the present invention that solves the above problems. Thesuperconductor composite material has coated HTS ceramic particles 50each having a noble metal coating 52. The coating 52 preferablycompletely surrounds each HTS ceramic particle 50. The noble metalcoating 52 is preferably metallic and electrically conductive. The HTSceramic particles 50 and coating 52 are disposed in a metal matrixmaterial 54. The metal matrix material has a λ greater than 0.2,preferably greater than 0.5, more preferably greater than 1.0. All elsebeing equal, the higher the λ of the metal matrix material, the better.The metal matrix material 54 is preferably selected from the materialslisted in Table 2, although it is understood that Table 2 does notnecessarily contain all the useful metal matrix materials. Any metal ormetal alloy with a high enough λ, adequate ductility, and compatiblechemical properties is suitable for use as the metal matrix material.

The HTS ceramic particles preferably have dimensions (not including thecoating 52) larger than the superconducting coherence length of the HTSceramic material. Typically, HTS ceramic materials have coherencelengths of about 1.5-3 nanometers, so the HTS ceramic particlespreferably are at least this large. The HTS ceramic particles havedimensions of about 5-500 nanometers. Preferably, the HTS ceramicparticles have dimensions of about 3-1000 times the superconductingcoherence length of the HTS ceramic, or more preferably, about 3-50times the coherence length. The best size range depends upon thetemperature at which the composite material is used, and the λ andproximity effect decay length of the metal matrix material, among otherfactors.

The noble coating 52 is preferably made of a noble metal that does notreact (i.e., is not oxidized) by contact with the HTS ceramic particles.Preferably, the coating is made of silver, although other metals inTable 6 can be used, as well as alloys of these metals. The metal matrixcan include alloys comprising metal s not listed in Table 6. Forexample, alloys of silver or gold with relatively more reactive metalsmay be nonreactive with the HTS ceramic. The noble coating 52 serves toprevent chemical reactions (e.g., oxidation) from occurring between theHTS ceramic particles and the metal matrix material 54. The noblecoating 52 should be as thin as possible while thick enough to preventchemical reactions between the HTS ceramic particles and metal matrixmaterial 54. Preferably the noble coating is about 5-50 nanometersthick, but the noble metal coating can also be as thick as 3000nanometers. Thick noble coatings negatively impact or adversely affectthe superconducting properties (e.g., Jc, Tc) of the compositesuperconducting material.

Silver is the preferred noble metal because it is the least expensive ofthe metals not oxidized by contact with HTS ceramics. Silver is alsopreferred because silver oxide is unstable at the modest temperaturesused to anneal the HTS ceramic material, further inhibiting theformation and persistence of an oxide layer.

Also, silver is permeable to oxygen at elevated temperature. This isbeneficial because the HTS ceramics require a high oxygen content forsuperconductivity. If oxygen is depleted from the HTS ceramic material,superconductivity is degraded. HTS ceramic particles coated with silvercan be replenished with oxygen because silver is permeable to oxygen atelevated temperature. The oxygen content of coated HTS ceramic particlesis restored by heating the coated particles in an oxygen atmosphere.

Preferably, the noble metal coating is thinner than the inelasticelectron man free path (MFP) in the noble metal at the criticaltemperature of the superconducting particles. More preferably, the metalcoating is thinner than ½ or ¼ of the electron MFP of the coating at thecritical temperature of the superconducting particles.

A long MFP allows electrons and holes from the HTS ceramic particles totravel a long distance in the noble metal. This increases theprobability that electrons and holes will reach the metal matrixmaterial without collisions and thereby provide a substantialsuperconducting gap magnitude in the metal matrix material. Silver isalso preferred because of its relatively long MFP. Again, it isemphasized that the noble metal coating should be as thin as possiblewhile still providing chemical isolation for the HTS ceramic particles.

Also, it is preferable for the noble metal coating to be thinner thanthe proximity effect decay length of the noble metal at the criticaltemperature of the superconducting particles. Proximity effect decaylengths are typically shorter than the MFP for noble metals, and theproximity effect decay length is inversely proportional temperature. Theproximity effect decay length is determined by the rate of dephasing ofelectron and electron-hole wave functions in the noble metal. Thedephasing is caused by differences in the electron and hole velocities.The proximity effect decay length is known and understood in the art.Table 7 gives the proximity effect decay lengths for some noble metals77 Kelvin.

TABLE 7 Proximity Effect Decay Lengths for Some Noble Metals Noble MetalProximity effect decay length at 77 K Silver 22 nm Gold 22 nm Palladium27 nm

FIG. 17 illustrates the operation of the embodiments employing HTSceramic particles 50 with a noble metal coating 54. When cooled belowthe HTS ceramic Tc, a proximity effect extends into the metal matrixmaterial 52 a certain range illustrated by the circles 36. It isunderstood that the proximity effect extends through the noble metalcoating 54 and into the metal matrix material 52 according to the modelexplained with reference to FIG. 15. The superconducting gap rebounds inthe metal matrix material 52. The proximity effect decays with distancefrom the HTS particles 50 so that the circles provide an arbitrarymeasurement of the proximity effect range. The particles 50 are closeenough together so that the circles 36 overlap, thereby providing acontinuous supercurrent path 38. The overlapping circles means that theparticles 50 are coupled by the continuous superconducting path due tothe proximity effect. Circle size increases with decreasing noble metalcoating thickness.

A method for preparing the HTS ceramic composite superconductor materialbegins with providing clean HTS ceramic particles of appropriate sizes.The HTS ceramic particles are then coated with a thin uniform coating ofnoble metal, preferably silver. Silver can be deposited using a numberof techniques known in the art such as chemical deposition and vapordeposition. Vapor deposition can be performed by sifting the particlesin a vacuum chamber having a partial pressure of silver, for example.Other techniques for forming the noble metal coating are known in theart.

After the HTS ceramic particles are coated with silver, the interior ofthe HTS ceramic particles can be replenished with oxygen. Replenishmentis performed by heating the coated particles in an atmosphere with apartial pressure of oxygen. Since silver is permeable to oxygen atelevated temperatures (300° C. and up), oxygen reaches the ceramic. Thebest temperature, oxygen pressure, and annealing time is specific toeach HTS ceramic and is selected to optimize the superconductingproperties of each ceramic. Such annealing techniques for silver coatedhigh Tc ceramics are well known in art. The coated HTS ceramic particlesare then thoroughly mixed with particles of the metal matrix material.The ratio of HTS ceramic particles to metal matrix material particlesdetermines the average spacing between the HTS ceramic particles. Themixing ratio has a large effect upon the superconducting properties ofthe composite material and should be optimized for a particularapplication. To form the composite into a wire, the mixture is thendisposed in a metal billet and compressed to fuse the mixture into adensely packed composite material. Compression may be performed undervacuum so that void space is minimized. The densely packed compositematerial is then drawn into a wire using conventional techniques. Ofcourse, the ceramic particles and metal matrix particles can becompressed to form any other shape such as bars, rods, sheets, orplates.

Alternatively, the coated HTS ceramic particles are coated with themetal matrix material. The twice coated HTS ceramic particles are thencompressed in a billet and drawn into a wire.

The present invention provides a new class of superconducting compositematerials that are designed to maximize the superconducting proximityeffect. The metal matrix material is selected based on itselectron-phonon coupling coefficient λ, and its chemical compatibility.In cases where a chemical incompatibility exists between the intrinsicsuperconductor particles and the metal matrix material (as in the caseof the HTS ceramics and the Nb₃Sn/lead combination), a noble metalcoating protects the superconductor particles. In the case of the HTSceramics, the metal coating is preferably a noble metal coating thatresists oxidation. For many other superconductor particles, the coatingcan be any metal compatible with other materials in the composite. Ifthe metal coating is thin enough, and has a long enough MFP, theproximity effect causes the surrounding metal matrix material to becomesuperconducting due to the proximity effect.

It is noted that the present invention includes many possiblecombinations of superconductor particles materials and metal matrixmaterials. Any ceramic superconductor particles can be combined with anyhigh-λ metal matrix material. If a chemical incompatibility exists inthe combination (e.g., the combination causes degradation of thesuperconductor particles or degradation of the metal matrix material, oran insulating coating forms at the superconductor particle/metal matrixinterface), then a chemically-insulating, electrically conductivecoating should be provided between the superconductor particles and themetal matrix material. The coating is preferably a noble metal if theceramic particles are highly reactive, such as many of the HTS ceramics.

For superconductor particle materials that are less reactive than theHTS ceramics, the coating can be made of high λ metals that are readilyoxidizable. However, the coating is preferably non-reactive with theparticles and the metal matrix material.

The present invention is not limited to the superconductor particlematerial and metal matrix materials listed herein.

It is not necessary in the present invention to only select materialsthat do not react at all with one another. A certain amount ofreactivity can be tolerated between the different components if thereactivity does not significantly damage the superconducting propertiesof the composite. For example, Nb₃Sn/lead composite demonstrates someamount of damaging reactivity, but, Nb₃Sn/lead composite is still usefulas a superconductor for certain applications and is within the scope ofthe present invention. The present invention covers any materialcombinations that do not destroy superconductivity in one of thematerial components, or does not form electrically insulating layers(e.g., metal oxides) at interfaces between the components.

Wire Manufacture

FIG. 18 to FIG. 25 illustrate how the materials hereinbefore describedcan be used in manufacturing a three-component wire, according to anembodiment of the invention.

Referring firstly to FIG. 18, a sheet 100 of ductile protective materialis provided on which superconductor particles 102 are deposited. Thesheet 100 may for example be made of any one of the materials listed inTable 3. The sheet 100 is initially relatively thick for ease ofhandling and to prevent tearing thereof. The sheet 100 may, for example,be made of silver having a thickness of about 0.05 mm.

The superconductor particles 102 are in granular form and can be made ofany one of the HTS materials listed in Table 5.

As shown in FIG. 19, the superconductor particles 102 are spread evenlyover a surface provided by the sheet 100. In FIG. 20, another sheet 104of the same material as the sheet 100 is located over the superconductorparticles 102. The sheet 104 typically has the same thickness as thesheet 100. The superconductor particles 102 are thereby located betweenthe sheets 100 and 104 of protective material.

Next, as shown in FIG. 21, a sheet 106 of ductile conductive material islocated on the sheet 104 of protective material 104. The sheet 106 maybe made of any one of the materials in Table 2. The sheet of protectivematerial 100, the superconductor particles 102, the sheet of protectivematerial 104, and the sheet of conductive material 106 thereby form fourlayers of a composite sheet 110. The composite sheet 110 has a thickness112, a width 114, and a length 116 (only a portion of which is shown).The thickness is typically about 1 cm, the length 112 about 1 m, and thewidth 114 about 20 cm.

The composite sheet 110 has a strip 118 at an edge thereof extendingalong the length 116 thereof. Another strip 120 is located next to thestrip 118, also extending along the length 116, and a further strip 122is located next to the strip 120, the strip 122 also extending along thelength 116. The strip 118 is folded on to the strip 120. The strip 118is thereby located on top of the strip 120. Folding of the strip 118onto the strip 120 is allowed for due to ductility of the sheets 100,104, and 106, and due to the superconductor particles 12 being insingulated granular form. The strip 118 is then folded onto the strip122 so that the strip 118 is located between the strip 122 and the strip120. The strips 118, 120, and 122 being folded or rolled onto oneanother are shown in FIG. 22. Once folded, the composite sheet 110 formsan elongate member 126 having a length 116, a height 128, and a width130. The height 128 is about 4 cm and is substantially the same as thewidth 130. Because of folding of the composite sheet 110, the elongatemember 126 has a height 118 and a width 130 which are smaller than thewidth 114 of the composite sheet 110. It should be understood that,although the composite sheet 110 is rolled in the embodiment described,other methods of forming the composite sheet 110 may provide a similarelongate member. The composite sheet 110 may for example, be fan-folded.

The components of the elongate member 126 are located relative to oneanother so that the superconductor particles 102 are located next to theconductive material 106 on one side thereof and located next to theconductive material 106 on opposing side thereof. The conductivematerial 106 on each side is separated from the superconductor particles102 by a respective portion of either the sheet 100 of protectivematerial or the sheet 104 of protective material.

FIG. 23 illustrates further processing of the elongate member 126wherein the elongate member 126 is rolled into a wire 136. The elongatemember 126 is rolled by rollers 138 and other known equipment whichreduce the dimensions of the elongate member 126 with a correspondingincrease in length thereof. FIG. 24 is a cross-section of the elongatemember 126 and FIG. 25 is a cross-section of the wire 136. As mentionedpreviously, the sheets 100 and 104 are initially relatively thick. Oncethe dimensions of the elongate member 126 are reduced to form the wire136, the sheets 100 and 104 also reduce in thickness. The thickness ofthe sheets 100 and 104 of the wire 136 are about ten times the decaylength of the material thereof. The superconductor particles 102 caninduce the conducting material 106 to a superconductive state throughthe sheet 100 or 104. The exemplary 1 m by 4 cm elongate member 126 mayfor example be drawn into wire 136 having a diameter of 1 mm, a lengthof 1.6 km and have sheets 100 and 104 of silver that are reduced to 0.05mm in thickness.

FIG. 26 is a cross-sectional view on 26-26 of FIG. 25. It can be seenthat the sheets 100, 104, and 106, and the superconductor particlesextend continuously through the wire 136. FIG. 26 only shows a sectionof the wire 136 but it should be understood that the wire 136 may bekilometers long with a continuous construction as shown in FIG. 26. Itshould, in particular, be noted that the conductive material 106 extendsthrough continuous kilometers of the wire 136.

A respective superconductor particle 102A induces a region of theconductive material 140A to a superconductive state through the sheet100. Another superconductor particle 102B induces another region 140B inthis conductive material 106 to a superconductive state. The region 140Boverlaps the region 140A, thereby providing a superconductive link fromthe region 140A to the region 140B. In a similar manner, anothersuperconductor particle 102C induces another region 104C of theconductive material 106 to a superconductive state, and subsequentregions are also induced to a superconductive state. The regions overlapone another so that an unbroken superconductive path in the conductivematerial 106 is provided through the entire length of the wire 136. Thewire 136 can thus conduct current in a superconductive nature when thewire 136 is cooled to an appropriate temperature wherein thesuperconductor particles 102 are superconductive.

The embodiment described with reference to FIG. 18 to FIG. 26 is for athree-component wire including (i) an HTS superconductor material fromTable 5, (ii) a conductive material from Table 2, and (iii) a sheet ofprotective material from Table 3 located between the HTS superconductormaterial and the conductive material. Such a wire allows for theproximity effect to be employed utilizing a reactive or brittle HTSsuperconductor material.

In another embodiment an intrinsic superconductor material from Table 1can be used together with a conductive material from Table 2. Theintrinsic superconductor materials of Table 1 have the advantage thatthey are not as reactive as HTS superconductor materials, so that a wireembodiment of an intrinsic superconductor material and a conductivematerial can be formed without a sheet of protective material betweenthe intrinsic superconductor material and the conductive material. Sucha wire is shown in FIG. 27 and can be formed by spreading particles 182of brittle superconductor material over a sheet 184 of ductileconductive material and then folding or rolling the sheet 184 ofconductive material into an elongate member so that the particles 182 ofsuperconductor material are trapped between successive layers of thesheet 184 of ductile material, whereafter the elongate member is drawninto the wire 180. The wire 180 requires fewer components while stillhaving a conductive material with an electron-phonon couplingcoefficient of at least 0.2.

FIG. 28 is a cross section on 28-28 in FIG. 27. It can be seen from FIG.28 that the particles 182 of superconductor material are locateddirectly against the sheet 184 of conductive material. The conductivematerial of the sheet 184 is driven to a superconductive state by thesuperconductor particles 182 without a protective sheet located betweenthe particles 182 and the sheet 184.

FIG. 29 to FIG. 37 now illustrate three different embodiments of makingthree-component powder-in-tube wire from the HTS materials listed inTable 5, the protective materials listed in Table 3, and the conductivematerials listed in Table 2, and FIG. 38 to FIG. 43 illustrate twoembodiments of making a two-component powder-in-tube wire utilizing theintrinsic superconductor materials of Table 1 and the conductivematerials of Table 2.

The first embodiment of making a three-component powder-in-tube wire isshown in FIG. 29 to FIG. 31. FIG. 29 shows one granule 200 that is usedin the manufacture of the wire. The granule 200 includes asuperconductor particle 202, a layer of protective material 204, and alayer of conductive material 206. The superconductor particle 202 may bemade of any one of the materials in Table 5. The layer 204 is formeddirectly on the superconductor particle 202 and entirely surrounds thesuperconductor particle 202. The layer 204 may be made of any one of thematerials in Table 3. A layer 206 is formed directly on and entirelysurrounds the layer 204. The layer 206 may be made of any one of thematerials in Table 2.

FIG. 30 illustrates an elongate member 208 that is used for rolling wireout of. The elongate member has a circular pipe container 210 that isfilled with a plurality of the granules 200. End caps 212 close offopposing ends of the pipe container 210. The elongate member 208 has alength 214, a width 216, and a height into the paper. The height and thewidth substantially equal one another. The layers 206 of conductivematerial of the granules 200 contact one another when located within thepipe container 210. The elongate member 208 is drawn in to a wire in amanner similar to that shown in FIG. 23. Drawing of the elongate member208 into wire causes a reduction in the width 216 and the height with acorresponding increase in the length 214. The elongate member 208initially has dimensions similar to the elongate member 126 in FIG. 22and drawn into wire having dimensions similar to the wire 136 of FIG.25.

FIG. 31 illustrates a core of wire 212 that is drawn out of the elongatemember 208. The layers 206 of the granules 200 of FIG. 29 are deformedso that they fill regions between the superconductor particles 202. Whatremains of the initial granules 200 are a plurality of coated particles214 with each coated particle 214 including a respective superconductorparticle 202 and a respective layer 204 of protective material. Thelayers 206 of conductive material forms a matrix 206A filling regionsbetween the coated particles 214. Deformation of the conductive layer206 is allowed for due to ductility of the material that they are madeof. A respective superconductor particle 202 induces or drives arespective region of the matrix material 206A surrounding the respectivesuperconductor particle 202 to a superconductive state. A respectiveregion given to a superconductive state by one of the superconductorparticles 202 overlaps another region given to a superconductive stateby another superconductor particle 202 so that a superconductive link iscreated from the one region to a next region. These regions overlap oneanother along an entire length 218 of the wire 212. A continuous chainof regions that are driven to a superconductive state is thereby createdalong the entire length 218 of the wire 212, which may be kilometerslong.

FIG. 32 to FIG. 34 illustrate a second embodiment of making athree-component powder-in-tube wire according to the invention. FIG. 32illustrates a granule 230 including a superconductor particle 232 and alayer 234 of protective material. The superconductor particle 232 ismade of a material from Table 5 and the layer 234 is made of a materialfrom Table 3. FIG. 33 shows an elongate member 236 assembled from anelongate pipe container 238 with a plurality of the granules 230therein. Opposing ends of the pipe container 238 are closed with endcaps 240. The pipe container 238 is made of a material from Table 2.

FIG. 34 illustrates a portion of a wire 242 that is drawn from theelongate member 236. The material of the protective layers 234 of thegranules 238 is sufficiently ductile so as to deform into a matrix 234Athat fills spaces between the superconductor particles 232. The pipecontainer 238 is also sufficiently ductile so as to deform into a casing238A that entirely surrounds the matrix material 234A and thesuperconductor particles 232. The superconductor particles 232 locatedclosest to an inner wall of the casing 238A are spaced from the innerwall of the casing 238A by a respective portion of the matrix 234. Thesuperconductor particles 234 are, however, sufficiently close to thecasing 238A so as to drive respective regions of the material of thecasing 238A to a superconductive state. These regions overlap oneanother so as to form a continuous chain of superconductive regionsextending throughout the wire 242.

FIG. 35 to FIG. 37 illustrate a third embodiment of making athree-component powder-in-tube wire according to the invention. As shownin FIG. 35, granules of superconductor particles 260 are used but areuncoated, i.e., having all external surfaces exposed. FIG. 36illustrates an elongate member 260 including an external pipe container264 of conductive material, an inner container 266, a plurality of thegranules 260, and end caps 268. In this embodiment, the outer pipecontainer 264 is made of a conductive material from Table 2 and theinner pipe container 266 is made of protective material from Table 3.The inner pipe container 266 is located within the outer pipe container264 as to form a layer covering an entire inner surface of the outerpipe container 264. The superconductor particles 260 are then insertedwithin the inner pipe container 266. The superconductor particles 260are entirely surrounded by the inner pipe container 266 and the innerpipe container 266 is entirely surrounded by the outer pipe container264. The end caps 268 are located over opposing ends of the pipecontainers 264 and 266. The exposed surfaces of the superconductorparticles 260 contact one another and some of the exposed surfacescontact an inner surface of the inner pipe container 266.

FIG. 37 illustrates a section of a wire 272 that is drawn from theelongate member 262. The thicknesses of the pipe containers 264 and 266are reduced to form elongate casings 264A and 266A respectively. Some ofthe superconductor particles 260 contact the elongate casing 266A butare prevented from contacting or reacting with the elongate casing 264A.The elongate casing 264A has a wall thickness that is sufficiently thinthat a respective one of the superconductor particles 260 can induce aregion of the material of the elongate casing 264A to a superconductivestate. These regions are linked in a chain throughout the entire lengthof the wire 272.

FIG. 38 to FIG. 40 illustrate a first embodiment of making atwo-component powder-in-tube wire utilizing an intrinsic superconductorfrom Table 1 together with a conductive material from Table 2. FIG. 38illustrates a granule 300 which includes a superconductor particle 302with a layer of conductive material 304 formed thereon. Thesuperconductor particle 302 is made of an intrinsic superconductormaterial from Table 1 and the layer 304 is formed directly onto andentirely surrounds the superconductor particle 302 and is made of aconductive material from Table 2.

FIG. 39 shows an elongate member 304 including an elongate pipecontainer 308, a plurality of the granules 300 within the pipe container308, and end caps 310 closing off ends of the pipe container 308. FIG.40 illustrates a core of a wire 314 that is drawn out of the elongatemember 306. The conductive material 304 is sufficiently ductile so as todeform and form a matrix 304A that fills regions between thesuperconductor particles 302. The superconductor particles 302 driveregions of the matrix 304A to a superconductive state and these regionsare linked to form an unbroken chain throughout the entire length of thewire 314.

FIG. 41 to FIG. 43 illustrate a second embodiment of forming atwo-component powder-in-tube wire from an intrinsic superconductormaterial from Table 1 and a conductive material from Table 2. FIG. 41illustrates superconductor particles 320 that are uncoated, i.e., havingbare outer surfaces. As shown in FIG. 42, the superconductor particles320 are located within an elongate pipe container 322 and ends of theelongate pipe container 322 are closed off with end caps 324 to completeassembly of an elongate member 326. In this embodiment, the elongatepipe container 322 is made of a conductive material from Table 2 and thesuperconductor particles 320 are made from an intrinsic superconductormaterial from Table 1.

FIG. 43 illustrates a section of a wire 332 that is drawn from theelongate member 326. The elongate pipe container 322 reduces inthickness to form an elongate casing 322A with the superconductorparticles 320 therein. The superconductor particles 320 drive regions ofthe material of the elongate casing 322A to a superconductive state.

Magnesium Diboride as a Superconductor Material

It has been found that magnesium diboride (MgB₂) displayssuperconductive properties at 40K, a temperature at which helium is avapor. It is also believed that magnesium diboride can be combined withany one of the conductive matrix materials in Table 2 without attackingthe conductive matrix material. There is thus no need for anintermediate protective layer. Particles of the magnesium diboride canbe located in direct contact to a conductive material discussed withreference to FIG. 1 or 43.

It may be possible to use metallic borides other than magnesiumdiboride. Other metallic borides such as planar diborides may prove tobe useful.

It may also be that a desirable conductive material is attacked bymagnesium diboride or another planar diboride. Such an embodiment mayrequire a protective material between the superconductive material andthe conductive material.

Gallium-based Superconductor/Metal Matrix Composite

A superconductor/metal matrix composite (SMMC) may be used which usesgallium or a gallium-based alloy as a ductile matrix metal which isdriven to superconductivity by the proximity effect.

High current density SMMC wires and tapes can be fabricated using metalmatrix materials that are chemically compatible with the superconductorparticles and that possess a high lambda. In general, the higher thelambda (λ), the higher the current-carrying capacity of the finalcomposite.

Gallium metal and its alloys are of particular interest because of theunique properties of gallium itself. In particular:

1) Gallium is a liquid at approximately 30° C. and has a tendency tosupercool. This allows for a very uniform distribution of the gallium orgallium-based alloy throughout the SMMC. This may be achieved, forexample, by ball milling, planetary milling, or attrition milling of agallium or the gallium-based alloy with an appropriate superconductingpowder.

2) Gallium or gallium-based alloys possess very high lambda values. Theelectron-phonon coupling in these materials is known to be extremelyhigh (>2). This results in a very high proximity-induced gap within themetal matrix and very high critical currents.

Gallium is also known to exhibit polymorphism. The different crystalstructures of this material have different lambda values, and thegallium or gallium-based metal matrix can be prepared to possesssubstantially the form of gallium or gallium-based alloy that has thehighest lambda.

In particular, it is well-known that amorphous or disordered materialspossess higher lambdas than their more ordered counterparts. Gallium andits alloys can easily be made to be substantially amorphous, thusincreasing lambda and the magnitude of the induced gap in the metalmatrix.

SMMC wires with an amorphous, high-lambda metal matrix will have highercritical current densities than SMMC wires with substantiallycrystalline metal matrix materials.

3) Gallium and gallium-based alloys are well-known to wet the surface ofmany materials including ceramics. This ability to adhere to the surfaceof ceramic or other brittle superconducting materials increases thetotal superconductor/metal surface area of the composite and since themagnitude of the proximity effect is proportional to the surface area,metal matrix materials that wet the surface of the superconductor willmake higher current carrying capacity SMMC based wires.

A 20% by vol. gallium or gallium-based SMMC wire may be fabricated usingthe following method:

1) 2.54 grams of MgB2 superconducting powder and 2.32 grams of liquid orsolid Ga metal are combined in a planetary ball mill (80 ml vial) with20 Si3N4 balls (10 mm diameter).

2) The composite powder is milled for a total of 4 hours, at 300 RPM. Aprocess control agent may be used during the milling process if there isexcessive cold welding during the mill. The use of process controlagents is well-known in the field of mechanical alloying.

3) The milled powder is then loaded into a copper billet and sealed. Thebillet itself can be materials other than copper, such as niobium,silver, iron, nickel or any other material that is compatible with theSMMC composite and which lends itself to the deformation process.

4) The packed billet is drawn or rolled to a final geometry usingwell-known wire fabrication methods. Single or multifilament wire ortapes may be made by these methods.

The final conductor consists of a metallic sheath (usually copper) andfilament(s) of a magnesium diboride/(gallium or gallium-based) metalmatrix composite.

FIG. 44 shows the increased performance of the gallium-based SMMC taperelative to Indium-based SMMC tape. Both tapes use MgB2 and thesuperconducting material, but it is clear that the gallium-based SMMCconductor has much higher engineering current densities than theIndium-based SMMC.

Gallium or gallium-based conductors may be made in a variety ofgeometries (e.g., single multifilament, round wire or tape) with avariety of superconducting powders.

The only requirement is that the superconductor/(gallium orgallium-based) metal interface be substantially non-reactive withrespect to the formation of an insulating barrier such that theproximity effect may exist in the (gallium or gallium-based) metal layeradjacent to the superconductor.

Depending on the superconductor, the gallium-based metal, and thecomposite preparation conditions, these procedures may need to becarried out under inert atmospheric conditions.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative and not restrictive of the current invention, andthat this invention is not restricted to the specific constructions andarrangements shown and described since modifications may occur to thoseordinarily skilled in the art.

1. A superconductor, comprising: particles made of a magnesium diboridesuperconductive material; and a conductive material selected to bedriven to a superconductive state when in proximity to thesuperconductive material and at least including gallium, an unbrokensection of the conductive material being located sufficiently close to aplurality of the particles to be driven to a superconductive state bythe superconductive material so that current conducts primarily throughthe conductive material of the superconductor.
 2. The superconductor ofclaim 1, wherein the conductive material is in contact with thesuperconductive material.
 3. A method of making a superconductor,comprising: mixing a plurality of particles of a magnesium diboridesuperconductive material with a conductive material, thereby locatingthe conductive material adjacent the superconductive material, theconductive material being selected to be driven to a superconductivestate when in close proximity to the superconductive material, theconductive material at least including gallium, and an unbroken lengthof the conductive material being in sufficiently close proximity to aplurality of the particles to be driven to a superconductive state bythe particles.
 4. The method of claim 3, further comprising: assemblingan elongate member from the particles and the superconductive material;and drawing the elongate member into a wire.
 5. A method of making asuperconductor, comprising: assembling an elongate member of aconductive material adjacent a plurality of particles of a magnesiumdiboride superconductive material, the conductive material beingselected to be driven to a superconductive state when in close proximityto the superconductive material, the conductive material at leastincluding gallium; and drawing the elongate member into a wire, anunbroken length of the conductive material being in sufficiently closeproximity to a plurality of the particles to be driven to asuperconductive state by the particles.